Definitions
The term "gap distance" (primary or secondary) herein used refers to the size reduction of the shape of one rotor and the term "ultimate gap distance" refers to the distance of the gap produced between two rotors.
The Roots type blower as shown in FIGS. 1 and 2 is a two-shaft type blower. The housing 1 has an inner space 2 which is peculiar to the Roots type blower and an intake port 3 and a discharge port 4 which are communicated with the inner space 2. Two shafts 5a and 5b are rotatably disposed in the inner space 2 of the housing 1 by support means such as bearings 6 and 6 so that a given spacing(gap) is provided between the shafts. The lower shaft 5a serves as an input shaft. The shafts 5a and 5b are rotated in the opposite directions by means of synchronizing gears 6a and 6b which are disposed outside the housing 1. Rotors 7a and 7b are secured to the shafts 5a and 5b so that they are in a phase difference of 90.degree. each other. The rotors 7a and 7b are rotated in a spaced relationship with each other and with the inner wall of the housing 1 so that they will not be interfered with each other. Usually such a gap is provided by reducing the shape of the rotors 7a and 7b such as a combination of epicycloidal and hypocycloidal curves by a given gap distance.
When the rotors 7a and 7b are rotated as shown in the drawings the two shaft type blower intakes air from the intake port 3 and imparts kinematic energy to the air in rotational directions of the rotors 7a and 7b within the inner space 2 of the casing 1 then discharges the compressed air from the discharge port 4.
As described above the prederermined gap distance is necessary for the rotors to avoid interfering with each other. The gap distance may be classified into the primary- and the secondary gap distance. The primary gap distance is necessary to provide a minimum gap distance between rotors for allowing them to rotate in a non-contact state. The secondary gap distance is necessary to prevent the interference with the adjacent rotor and the casing which occurs otherwise due to the tolerance is working and assembling of the parts such as rotors and the casing. It is the phase tolerance between the rotors that gives the greatest influence upon the determination of the secondary gap distance. This phase tolerance takes place mainly due to the assembly tolerance between the rotor and the shaft and meshing tolerance between synchronizing gears (including the assembling tolerance between the gear and the shaft). It is possible to somewhat reduce the phase tolerance by improving the precision of working and assembling the rotors and the synchronizing gears. The secondary gap distance is determined in anticipation to a possible maximum phase tolerance which is taken into consideration in the present state of the art.
In a conventional Roots type blower, the primary and the secondary gap distances have generally been provided by reducing the size of roots into a similar figure or equally reducing them in a direction normal to a corrected curve of the rotors. Such manners of determination of the gap distance will result in a resultant ultimate gap having a mean width at least as double as the designed gap distance. The mean width of the resultant clearance results in remarkably lowering the volume efficiency of the Roots type blowers.